We hope to define ontogenetically and phylogenetically the mechanisms of immunity using two principal animal groups: 1) anuran amphibians (frogs), the first terrestrial tetrapods, and 2) and advanced coelomate invertebrate (earthworms). The amphibian model will be used to study lymphocyte polyclonal expansion in adult frogs and larvae using mitogens in vitro. The role of temperature on activation and stimulation will be emphasized. These studies are significant, confirming that clonal expansion was an early evolutionary event in the phylogeny of immunity, and that the in vitro approach is an excellent tool for predicting what happens in vivo. Graft rejection in earthworms is a cell-mediated process accomplished by leukocytes (coelomocytes) whose cytoarchitecture is unknown. Earthworms possess a variety of coelomocytes as revealed by light and electron microscopy. Understanding their structure will make it possible to define some of the events in graft rejection. During the rejection of transplants in earthworms, biochemical events occur involving the synthesis of certain proteins (enzymes) associated with lysosomes. These enzymes, in particular, acid phosphatase, are found associated with coelomocytes and are free in the coelomic fluid. How coelomocytes mediate rejection is unknown but it is believed that acid phosphatase (and probably other enzymes) is involved. Coelomocytes are found in association with foreign grafts, but how they arrive must be determined. The working hypotheses exist: 1) recruitment or 2) division. Incorporation of tritiated thymidine into coelomocytes from night crawlers does not occur. Evidence using Boyden chambers showing chemotaxis favors the recruitment hypothesis. The significance of these biochemical approaches to the coelomocyte will extend our understanding of cell-mediated immunity in primitive animals and help confirm the hypothesis that cell-mediated immunity arose phylogenetically in the invertebrates. BIBLIOGRAPHIC REFERENCES: Brown, B.A., Wright, R.K. and Cooper, E.L. 1975 Lymphoid organs and amphibian immunity. Adv. Exp. Med. Biol. 64: 267-275. Cooper, E.L. 1975 Characteristics of CMI and memory in annelids. Adv. Exp. Med. Biol. 64: 127-136. Exceeds Capacity)